Superposition of A4 DBL 3X crystal structures (PDB codes: 2BQK and 3CPZ). (a) Superposition shown in surface representation. The A4 DBL 3X crystal structures are coloured brown (PDB code: 3BQK) and blue (PDB code: 3CPZ. (b) Close view of the sulphate-binding site. The sulphate ions bound to the structures are labelled S1 (green; PDB code: 2BQK) and S2 (red; PDB code: 3CPZ) respectively. S1 makes contacts with residues Lys1324, Gly1329 and Arg1467. The sulphate ion S2 however does not make protein residue contacts and the distance of its sulphur atom from the sulphur atom of S1 is ~3.6Å.